The invention lies in the printing technology field and pertains, more specifically, to a method of correcting local, machine-based inking errors on rotary printing machines, which can occur as stripe formation, ink fading or as ghosting in the finished printed product, reducing its quality.
In an offset printing process, because of the construction of the offset printing unit, it is not possible to ink a printed sheet completely uniformly or, in the case of a predefined subject, to ink the raster points with a constant layer thickness. Using an offset printing unit, the inking of a printing plate clamped onto the circumference of a plate cylinder is indeed carried out continuously with an ink film, which has been reduced to a specific ink film thickness by a multiplicity of distributor or bridging rolls. However, the ink uptake takes place discontinuously, since, firstly, the ink uptake depends on the subject and, secondly, the circumferential face of the plate cylinder accommodating the printing plate is discontinuous as a result of the clamping channels.
Disruption to the ink transfer, for example at the interface from the inking unit to the surface of the plate cylinder discharging the ink to the transfer cylinder, at the interface between the printing plate cylinder and the transfer cylinder surfaces and also at the interface between the surface of the transfer cylinder, which is most often formed as a rubber blanket covering, and the printing material to be printed leads to nonuniformities. These can have an effect both on the ink layer thicknesses transferred and, in relation to the raster areas, on the point size produced. The effects which result from this are stripe formation, ink fading in the printing direction and ghosting effects. The relationships between these phenomena which occur in the inking unit are quite complex and cannot be considered in isolation from one another.
In order to achieve high uniformity in the course of the ink density or the ink layer thickness in the print, these effects are taken into account to the greatest extent, in the context of simulation programs and design principles, in the design of inking units on rotary printing machines, and the inking unit is optimized in relation to the effects mentioned. Furthermore, there is the possibility of influencing these errors, for example by means of distributor rolls used in the inking unit and the point at which they start operating. In addition, the selection of suitable transfer covering surfaces functioning as transfer cylinder covers, such as rubber blankets, permits the occurrence of dynamically caused stripes to be reduced. However, the specific local correction of inking non-uniformities in the print cannot be achieved with these measures.
It is accordingly an object of the invention to provide a method of correcting inking errors in rotary printing machines, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows correcting machine-based inking errors locally.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of correcting local, machine-based inking errors in a rotary printing machine, which comprises the following method steps:
determining an inking behavior of a printing unit by means of test forms;
registering inking nonuniformities on the test forms and transmitting data concerning the inking nonuniformities to a pre-press stage; and
modifying raster tonal values of the color separations of a printing form such that the inking nonuniformities are anticipated locally based on a location of their occurrence in the print on the printing forms or printing films.
In other words, the method corrects local, machine-based inking errors on rotary printing machines by executing the following steps:
determining the inking behavior of the printing units by means of test forms;
registering the inking nonuniformities on the test forms and transmitting the data about inking nonuniformity to the pre-press stage; and
modifying the raster tonal values of the color separations of a printing plate in such a way that the inking nonuniformities are compensated locally on the basis of the location of their occurrence in the print on the printing plates or films. The advantages which result from the solution proposed according to the invention can primarily be seen in the fact that disruptive stripes in the finished printing product, the ink fading in the printing direction and ghosting effects are no longer visually perceptible, or only to a slight extent. It is already possible to take account of the printing or inking behavior of the printing plate in a machine-specific way in the pre-press stage, i.e., during make-ready, so that the nonuniformity effects which set in later during the inking of the surface of the printing plate can be taken into account during the production of the printing plate and are compensated for in the subsequent print. As a result of the method proposed according to the invention, machine-specific characteristics may be maintained not only in global form but locally on the basis of the location of their occurrence, as a function of the raster tonal values, and therefore the quality of the printed product to be produced may be increased significantly.
In accordance with an advantageous development of the method of the invention, the inking behavior of the printing units is determined by means of test forms for the maximum format. The determination for the maximum format offers the advantage of using the latter as a reference value or reference printed product and of being able to convert from the latter to smaller formats from the determined inking nonuniformities. In accordance with an additional feature of the invention, in the case of inking nonuniformities determined independently of printing units, the inking nonuniformity of the rotary printing machine can be determined on only one representative printing unit. This offers the advantage that only one of a number of printing units has to be chosen, so that, during the calibration within the pre-press stage, a time advantage may be achieved by evaluating only one printing unit. In the case of a multicolor printing machine, which, for example, comprises 4, 6 or 8 printing units connected in series one after another, this represents a significant time advantage.
In accordance with another feature of the invention, in the case of subject-dependent inking nonuniformities, the modification of the color separations of a printing plate, be it in plate form or as a printing sleeve or a film, can be carried out on the basis of the subject, using the data from the raster image processor. Subject-dependent inking nonuniformities primarily occur when regions with high raster tonal values and regions with extremely low raster tonal values are located close beside one another in the print. By means of the method proposed according to the invention, it is possible to take into account not only the printing-machine or printing-unit dependent behavior of the production of inking nonuniformities but also the inking nonuniformities which occur as a result of the subject to be printed because of extremely different raster tonal values.